In the past, as a means for purifying exhaust gas in automotive internal combustion engines, a three-way catalyst simultaneously promoting oxidation of incompletely burned components, that is, HC (hydrocarbons) and CO (carbon monoxide), and reduction of the NOx (nitrogen oxides) formed by reaction of the nitrogen in the air and the oxygen remaining unburned has been utilized. To raise the oxidation and reduction abilities of such a three-way catalyst, it is necessary to control the air-fuel ratio, which shows the combustion state of the internal combustion engine, to near the stoichiometric air-fuel ratio. For that purpose, in fuel injection control in an internal combustion engine, an O2 sensor (oxygen concentration sensor) sensing whether the exhaust air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio based on the residual oxygen concentration in the exhaust is provided and air-fuel ratio feedback control correcting the fuel feed amount based on that sensor output is performed.
In such air-fuel ratio feedback control, the O2 sensor for detecting the oxygen concentration is provided as much as possible at a location near the combustion chamber at the upstream side from the three-way catalyst. To compensate for fluctuations in the output characteristics of that O2 sensor, a double O2 sensor system further providing a second O2 sensor at the downstream side of the three-way catalyst is also realized. That is, at the downstream side of the three-way catalyst, the exhaust gas is sufficiently agitated. The oxygen concentration is also in a substantial equilibrium state due to the action of the three-way catalyst, so the output of the downstream side O2 sensor changes more gently than the output of the upstream side O2 sensor and shows the rich/lean tendency of the air-fuel mixture as a whole. The double O2 sensor system uses the catalyst upstream side O2 sensor for main air-fuel ratio feedback control and uses the catalyst downstream side O2 sensor for secondary air-fuel ratio feedback control. For example, by correcting the related constants in the main air-fuel ratio feedback control based on the output of the downstream side O2 sensor, fluctuations in the output characteristic of the upstream side O2 sensor can be absorbed and the precision of air-fuel ratio control can be improved.
Further, in recent years, an internal combustion engine using a three-way catalyst having an oxygen storage capacity and controlling the air-fuel ratio of the exhaust flowing into the three-way catalyst so that the three-way catalyst can constantly exhibit a certain stable purification performance has also been developed. The oxygen storage capacity of a three-way catalyst stores the excess amount of oxygen when the exhaust air-fuel ratio is in a lean state and releases the insufficient amount of oxygen when the exhaust air-fuel ratio is in a rich state to thereby purify the exhaust, but this capacity is limited. Therefore, to effectively use the oxygen storage capacity, it is crucial enable the exhaust air-fuel ratio to next become the rich state or lean state by maintaining the amount of oxygen stored in the three-way catalyst at a predetermined amount, for example, half of the maximum oxygen storage amount. If maintaining it in this way, a constant oxygen storage and release action becomes possible at all times and as a result constant oxidation and reduction abilities by the three-way catalyst are always obtained.
In an internal combustion engine controlling the oxygen storage amount to a constant level so as to maintain the purification performance of the three-way catalyst, for example, there is known an air-fuel ratio control system where air-fuel ratio sensors are arranged at both the upstream side and downstream side of the three-way catalyst, a linear air-fuel ratio sensor able to linearly detect the air-fuel ratio is arranged at the upstream side, and an O2 sensor outputting a different output voltage depending on whether the exhaust air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio is arranged at the downstream side. In that air-fuel ratio control system, the linear air-fuel ratio sensor arranged at the upstream side of the three-way catalyst detects the air-fuel ratio of the exhaust flowing into the three-way catalyst, the O2 sensor arranged at the downstream side of the three-way catalyst detects the air-fuel ratio state of the three-way catalyst atmosphere, the oxygen storage amount of the three-way catalyst is controlled to be constant by controlling the target air-fuel ratio of the exhaust flowing into the three-way catalyst based on the detection information of the O2 sensors, and the air-fuel ratio of the exhaust flowing into the three-way catalyst is controlled to that target air-fuel ratio by feedback control of the fuel injection amount based on the output information of the linear air-fuel ratio sensor (see specification of Japanese Patent Publication (A) No. 11-82114).